A nonwoven fabric is a preformed web or mat of fibers laid down mechanically. The fibers may be deposited in a random manner or oriented primarily in one direction. Most widely used fibers include cellulosics, polyamides, polyesters, polypropylene and polyethylene. The spun fibers, which may be drawn, are laid down directly onto a porous belt by carding, air-laying or wet-laying. The sheet is then bonded together with a binder subsequently treated in an oven or a calendar to complete the bonding process.
A number of methods have been developed for applying a binder to randomly-dispersed fibers. Typically, a water based emulsion binder system is used in which a thermoplastic or thermoset synthetic polymer latex is prepared and a loose preformed web of fibers to be treated is immersed therein, saturated or sprayed using special equipment in view of the structural weakness of the preformed web; the thus treated preformed web is dried and cured to effect proper bonding. Alternatively, an aqueous or solvent solution binder system of a thermoplastic or thermoset resin may be used to impregnate the fibrous preformed web.
Still other methods include the application of thermoplastic or thermoset resin powders to the fibers, before or after making a preformed web of the same, and passing the preformed web through hot rolls or a hot press to bind the fibers together. Alternatively, thermoplastic fibers having a softening point below that of the base fibers may be interdispersed in a preformed web of the latter and sufficient heat and pressure applied, such as by the use of heated rolls, to soften the thermoplastic fibers and bind the fiber network together.
Commonly used latices for non-woven fabrics are those prepared from polymers of butadiene-styrene, butadiene-acrylonitrile, vinyl acetate, acrylic monomers such as methyl acrylate, ethyl acrylate, methyl methacrylate and the like. While the emulsion binder system is the most popular method of forming non-woven fabrics, the homopolymers, copolymers and terpolymers heretofore used therein have suffered from one or more disadvantages. To be useful as a textile material, the synthetic polymer must possess several physical properties. The desired properties include adequate tensile strength over a fairly wide temperature range, a high modulus or stiffness under certain conditions, and good textile qualities such as tenacity, handle and drape.
Initially, the use of styrene/butadiene latices in nonwovens has generally been restricted to disposable applications because of their tendency to discolor and embrittle upon exposure to long periods of heat, light and gas. Moreover, it will be appreciated that it has been an accepted practice to use self crosslinking or melamine formaldehyde resin posted latices to give improved wet and solvent tensiles to a nonwoven non-cellulose product. These systems, however, contain and liberate formaldehyde during the dry/cure cycle. In addition, essentially all commercial self crosslinking and melamine posted latices require a temperature of at least 280.degree. F. and preferably 300.degree. F. for proper crosslinking. However, it will be appreciated that because the melting point of some non-cellulose fibers is below the temperature required for proper crosslinking, e.g., polypropylene is around 250.degree. F., conventional latices cannot be used. Accordingly, nonmelt bonded polypropylene fiber in the nonwoven industry has never enjoyed large success. The problem has been in the specific development of a suitable latex binder to give acceptable tensile properties.
The nonwovens in accordance with the present invention chemically bond at temperatures below the melt bonding point of the non-cellulose fibers. Accordingly, the nonwovens in accordance with the present invention provide reduced energy costs for nonwoven manufacturers due to lower temperature requirements and the ability to chemically bond the fibers. It is an object of the present invention to provide a nonwoven fabric of chemically bonded non-cellulose fibers. Another object of the present invention is to provide a nonwoven fabric including a random arrangement of non-cellulose fibers and an essentially formaldehyde free latex binder capable of developing maximum tensile properties at temperatures less than the melt bonding temperature of the non-cellulose fibers. It is another object of the present invention to provide a nonwoven fabric of chemically bonded non-cellulose fibers that is simple and economical to manufacture.